This is a continuation-in-part application of Ser. No. 08/514,119 filed Aug. 11, 1995 U.S. Pat. No. 5,639,373. Terms used in the parent case are summarized in a glossary herein to shorten the specification; and, to avoid repetition herein, additional details in the parent case as well as in provisional application Ser. No. 60/012,921 filed Mar. 6, 1996, are incorporated herein by reference thereto as if fully set forth herein. In particular, considerations relative to the prior art and details of operation of prior art devices, all of which have been set forth in the '119 parent and provisional applications, are incorporated herein by reference thereto as if fully set forth herein.
This invention relates to a membrane device which is an improvement on a frameless array of hollow fiber membranes and a method of maintaining clean fiber surfaces while filtering a substrate to withdraw a permeate, which is also the subject of U.S. Pat. No. 5,248,424; and, to a method of forming a header for a skein of fibers.
This invention is particularly directed to relatively large systems for the microfiltration of liquids, and capitalizes on the simplicity and effectiveness of a configuration which dispenses with forming a module in which the fibers are confined. As in the '424 patent, the novel configuration efficiently uses air discharged near the base of a skein to produce bubbles in a specified size range, and in an amount large enough to scrub the fibers, and to provide controlled scrubbing of fibers one against another (“inter-fiber scrubbing”). Unlike in the '424 system the fibers in a skein are vertical and do not present an arcuate configuration above a horizontal plane through the horizontal center-line of a header. As a result, the path of the rising bubbles is generally parallel to the fibers and is not crossed by the fibers of a vertical skein. Yet the bubbles scrub the fibers.
The restrictedly swayable fibers, because of their defined length, do not get entangled, and do not abrade each other excessively, as is likely in the '424 array.
The side-to-side displacement of an intermediate portion of each fiber within the “zone of confinement” or “bubble zone” is restricted by the fiber's length. The defined length of the fibers herein minimizes (i) shearing forces where the upper fibers are held in the upper header, (ii) excessive rotation of the upper portion of the fibers, as well as (ii) excessive abrasion between fibers. Such swaying motion of a fiber with side-to-side displacement is distinct from vibration which occurs when a fiber is taut, that is, when the length of the ported fiber exposed to substrate is not longer than the distance between the opposed faces of upper and lower headers holding the fiber. Such vibration is induced by bubbles in a process for exfoliating and precipitating dense particles in U.S. Pat. No. 5,209,852 to Sunaoka et al. Unlike the fibers held in the module used in the '852 process, in our novel skein, there is essentially no tension on each fiber because the opposed faces of the headers are spaced apart at a distance less than the length of an individual fiber.
The use of an array of fibers in the direct treatment of activated sludge in a bioreactor, is described in an article titled “Direct Solid-Liquid Separation Using Hollow Fiber Membrane in an Activated Sludge Aeration Tank” by Kazuo Yamamoto et al in Wat. Sci. Tech. Vol. 21, Brighton pp 43-54, 1989, and discussed in the '424 patent the disclosure of which is incorporated by reference thereto as if fully set forth herein. The relatively poor performance obtained by Yamamoto et al was mainly due to the fact that they did not realize the critical importance of maintaining flux by aerating a skein of fibers from within and beneath the skein. They did not realize the necessity of thoroughly scrubbing substantially the entire surfaces of the fibers by flowing bubbles through the skein to keep the fibers awash in bubbles. This requirement becomes more pronounced as the number of fibers in the skein increases.
Tests using the device of Yamamoto et al indicate that when the air is provided outside the skein the flux decreases much faster over a period of as little as 50 hr, confirming the results obtained by them. This is evident in FIG. 1 described in greater detail below, in which the graphs show results obtained by Yamamoto et al, and the '424 array, as well as those with a vertical skein in which the headers are rectangular, all three assemblies using essentially identical fibers, under essentially identical conditions.
The investigation of Yamamoto et al with downwardly suspended fibers was continued and recent developments were reported in an article titled “Organic Stabilization and Nitrogen Removal in Membrane Separation Bioreactor for Domestic Wastewater Treatment” by C. Chiemchaisri et al delivered in a talk to the Conference on Membrane Technology in Wastewater Management in Cape Town, South Africa, Mar. 2-5, 1992, also discussed in the '424 patent. The fibers were suspended downwardly and highly turbulent flow of water in alternate directions, was essential.
It is evident that the disclosure in either the Yamamoto et al or the Chiemchaisri et al reference indicated that the flow of air across the surfaces of the suspended fibers did little or nothing to inhibit the attachment of microorganisms from the substrate.
Later, in European patent application 0 598 909 A1 filed by Yamamori et al, they sought to avoid the problem of build-up on the fibers by “spreading the hollow fibers in the form of a flat sheet” (see page 4, lines 46-7) and there is no indication how the fibers would be maintained in a spread position in actual use. Further, each array is held in a “structural member for enclosing and supporting the fastening member” (see page 3, line 42, and lines 51-52) which is contrary to the concept of a frameless array. Their FIGS. 14, and 18 emphasize the horizontal configuration in which the array is used. To combat build-up FIG. 13 depicts how the fibers would trough when the array is taken out of the reservoir to be “vibrated” or shaken. A prior art module is illustrated in FIG. 16 showing both ends of each fiber potted in a cylindrical header, each fiber forming a loop, the looped ends being free. As the data in FIG. 17 shows, use of the prior art cylindrical module with looped ends freely movable in the substrate, was less effective than the frameless array with spread apart looped fibers shown in FIG. 1.